Absorption of gases



G. l.. MATEER ET A1.

ABSORPTION OE GASES Filed sept. xfs, 1936' WN ,L f AMRYN May 9, 1939.

All

Patented May 9, 1939 PATENT ori-ics ABSORPTIGN F GASES George L. Mateer, Roselle, and Walter Il. Rupp,

Elizabeth, N. J., assignors to Standard Oil Development Company, a corporation of Dela- Ware Application September 18, 1936, Serial No. 101,352

3 Claims.

The present invention relates to a process for the recovery of hydrocarbons boiling within the .gasoline range from-gas mixtures containing the same, by absorption of said hydrocarbons from n said mixtures by a suitable menstruum. It is particularly directed to an improvement in the method of recovering the gasoline hydrocarbons from the absorption medium.

In renery operations, such as in the stabilizam tion of naphtha, in cracking and in destructive distillation, large amountsfof gases are formed, composed of methane, ethane, propane, and the corresponding oleiines, and containing substantial quantities of higher hydrocarbons, such as butane, pentane, etc. Natural gases of similar composition are also obtainable. It is desirable to recover the YC4 and higher hydrocarbons from such gas mixtures, since these hydrocarbons may be employed directly in motor fuels. It is customary to subject such gas mixtures to absorption conducted under conditions suitable for the absorption of the Ciand higher hydrocarbons exclusively. The sharpness of the separation of C3 and C4 hydrocarbons'depends upon the partial pressure of these constituents. The higher the partial pressure. of C4 and higher hydrocarbons, the more of these is it possible to absorb without absorbing undesirably large amounts of C3 and lower hydrocarbons. The absorption medium, thus enriched in C4 and higher hydrocarbons, is subjected to a stripping operation, such as steam stripping, for the removal of these constituents therefrom.

It has already been proposed, in a proces's'of the type referred to above, to substitute the dry gas resulting from the absorption step for steam as a. stripping agent in a low pressure stripping column for, the absorption medium. 'I'his dry gas is composed mainly of C1, C2, and C: hydrocarbons. In 40 stripping the absorption medium, this dry gas picks up some pentane and butane and must be recycled to the absorption tower for the recovery ofl these desired hydrocarbons.

The recycling of the stripping gas, containing smaller amounts of C4 and higher'hydrocarbons than the initial gas mixture to be subjected to absorption, produces a gas mixture having a lower partial pressure of the desired hydrocarbons than the initial gas mixture itself. Since absorption is a function of partial pressure, it follows that in orderfor the absorption medium to absorb all the desired hydrocarbons from the mixture of initial gas and recycle gas, it must absorb a larger quantity of propane' and lighter gases thanfit would 55 have to absorb from the initial gas mixtureper se;

in order to remove the desired hydrocarbons from it. It follows that a much larger amount of absorption medium is required for the recovery of the desired hydrocarbons than would be required for the initial gas mixtureitself.

It has now been found that the chief advantage arising from the stripping of the absorption medium with dry gas, namely, a saving of steam, can be realized without the loss in capacity and selectivity of the absorption medium resulting from this procedure, and actually with an increase in capacity, that is, a. decrease in the amount oi absorption medium required for a given amount of initial gas, and, more important, with a substantial increase in the selectivity of the absorption medium on C4 and higher hydrocarbons in the absorption chamber. According to the present invention, the gas which is to be subjected to absorption is employed as the stripping agent for the absorption medium in the low pressure or high pressure still, or both. In stripping the absorption medium, this gas becomes enriched in Ct and higher hydrocarbons, whereby the partial pressure of `the latter in said gas mixture is increased. Consequently, it is possible to absorb the C4 and higher hydrocarbons fromthis enriched gas mixture by the absorption lmedium with a smaller absorption of C3 and lighter hydrocarbons than would be'the case with the initial gas mixture itself. Accordingly, the C4 and higher hydrocarbons are more selectively absorbed from this gas mixture than would be possible if the initial gas mixture were scrubbed prior to its useA as a stripping agent,

The absorption medium may be any relatively Ahigh boiling material known to have absorptiveV power for butane, pentane, etc. Generally, the medium employed is a petroleum oil which may boil within the range beginning with heavy naphtha and en'ding with gas oil. Kerosene or a light l gas oil is particularly suitable for this purpos'e.

A front elevation of an apparatus suitable for carrying outI the process of the 'present invention is` shown in diagrammatic ,form in the attached drawing. Referring to the drawing in detail,| designates an absorption column to which is fed a rich gas through line 2 and a lean absorption medium through line 3 from lean oil tank l. I'he fat oil leaves the bottom of the absorption tower through line 5 and the leangasleaves the tower through line 6, which conducts it to burners, or to any processing equipment such as a fractionator, in which the C3 hydrocarbons may be isolated and conducted to a polymerizing unit or an. alcohol plant, the remaining Ci and C: hydrocarbons being conducted to a hydrogen production unit.

The fat absorption medium passes through a heat exchanger 1 into a vent tank 8 from which a gas, mainly composed of Ca and C: hydrocarbons, passes of! through line 9through which it passes to join the initial gas feed, the resulting fat absorption liquid leaving tank through line Il and passing through a preheater I I into a high pressure still I2. 'I'he partially stripped absorption medium leaves still I2 through line I3, which conducts it to low pressure still- Il from which the lean absorption medium is conducted through line I to heat exchanger 1, then to water cooler I6, and thus back to the lean oil tank l.

'Ihe rich gas to be subjected to absorption enters the system through line I1 in which it is conducted through a preheater Il into the bottom of the high pressure still I2. A branch line II `conducts part of the rich gas through preheater 20 into the bottom of low pressure still I4. The vapors leaving stills I2 and Il are cooled by conventional water coolers 2I and discharged into separators III from which some of the higher hydrocarbons are drawn of! through lines 3| and introduced into the tops of the respective towers as lreux, and the residual gases and'recovered liquid hydrocarbons are passed through line 22 to a storage drum 23 from which the residual gases are pumped through line 2 to absorption tower I. Drum 23 is maintained under a pressure suitable for the liquefaction of C4 and higher hydrocarbons and is provided with a draw off line 23 for removing the recovered liquid hydrocarbons.

Although not shown on the drawing, a gas compressor and liquid pump is sometimes required to pump gas and liquid from low pressure separator 30 to storage drum 23.

Either one or both of the stills maybe provided with a steam line2l through which steam may be introduced to facilitate the stripping operation. Likewise each still is provided with a reboiler 25, of conventional design, by which the desired temperature is maintained in the bottom of the still. Feed line I1 is provided with a branch 26 connecting it with line 2, whereby excess feed gas above stripping requirements may be fed directly to the absorption tower. Lines I1, I9 ahd26 are provided with valves 21 which mayxbe manipulatedso as to send all or part o! the initial rich gas to either still or to the absorption column. It may be pointed out here that in actual operation the bulk of the gas is sent to the low pressure still Il.

In operation, the rich gas enters the system at approximately atmospheric temperature and under a pressure of about 130 lbs/sq. in gauge. The high pressure still is maintained at about 80 lbs/sq. in and the low pressure still at about 10 lbs/sq. in. Upon leaving the stills, thegas, now further enriched in C4 hydrocarbons, is cooled to water temperature, which is usually about 80 F. and may be between 50 and 90 F., at which `temperature it enters the bottom of the absorption tower. The lean oil enters the top of the absorption tower at substantially the same temperature as the rich gas. I'his tower is usually maintained under a pressure of about 75 lbs/sq. in. i

Due to heat of absorption, the fat oil leaves the bottom of the tower at a temperature ranging from 100 to 140 F. In heat exchanger 1 this temperature 'is brought to between 250 and 290 F., and in preheater I I to a temperature between 305 and 345 F. at which it enters high pressure still I2. The temperature at the bottom of this still is held at between 300 and 340 F. Bubstantially the same temperatures are held in the low pressure stillfrom the bottom of which the lean oil leaves at a temperature between 300 and 340 F. which, in heat exchanger 1, is reduced to a temperature between 110 and 150 F. and in water cooler I6 to between 50 and 90 l".

The nature and objects of the present invention having been thus described, what is claimed as new and useful and desired to be secured by Letters Patent is:

1. An improved process for recovering low boiling liquid hydrocarbons from gaseous mixtures containing the same, which comprises the steps of passing the gaseous mixture into a still in countercurrent relation toa rich absorbent oil containing both gaseous and normally liquid hydro-K carbons, maintaining the still under superatmospheric pressure and at a 4temperature which is adapted to cause removal of the major part of the normally gaseous hydrocarbons while retaining the major quantity of C4 and higher hydrocarbons, removing the partly stripped oil to a separate stripping zone maintained at a lower pressure and at a temperature adapted to vaporize the normally liquid absorbed constituents from the oil, separately condensing the overhead vapors from the two stills, withdrawing the uncondensed vapors, scrubbing the same with cooled stripped oil derived from the low pressure still and returning the oil to the high pressure still.

2. A process for separating hydrocarbons having at least fourl carbon atoms to the molecule from a gas mixture containing them in addition to C1 to C: hydrocarbons, which comprises the gas mixture to a still maintained under superatmospheric pressure in stripping relation to' an absorption medium lich in hydrocarbons having at least four carbon atoms to the molecule, discharging the partially stripped oil into a lower pressure still whereby further quantities of hydrocarbons having at least four carbon atoms to the molecule are vaporized, subjecting the overheads from the two stills to partial condensation, feeding the combined uncondensed gas released i'rom the overheads to the bottom of an absorption zone, passing the lean oil to the top oi' said absorption zone from the second still, recovering fat oil from the bottom of said absorption zone and introducing it into the rst still, the 'lean oil passing thru a heat exchanger in heat exchange relation to the fat absorption medium leaving the absorption zone whereby hydrocarbons having not over three carbon atoms to the molecule are forced out of solution in said fat absorption medium, leading oil.' C; to C: hydrocarbons from the top of said absorption zone a'nd recovering hydrocarbons having at least i'our carbon atoms tothe molecule in each cycle\by partial condensation of the overhead from each still prior to the passage oi.' said combined uncondensed overhead to the absorption zone.

3. A process for separating hydrocarbons having at least four carbon atoms to the molecule from la gas mixture containing them in addition to Ci to C3 hydrocarbons, which comprises passing a portion of the gas mixture into a. still maintained under superatmospheric pressure in stripping relation to an absorption medium rich in hydrocarbons having at least four carbon atoms to the molecule, discharging the partially stripped oil into a lower pressure still into which the remalnder of the initial gas mixture is introduced in stripping relation to the absorption medium fed thereto, whereby further quantities of hydrocarbons having at least four carbon atoms to the molecule are vaporized, subjecting the overheads from the two stills to partial condensation, feeding the combined uncondensed gas released from the .overheads Ato the bottom of an absorption zone, feeding the lean oil from the second still to the topv of said absorption zone, recovering fat oil from the bottom of said absorption zone and introducing into the first still, leading off C1 to C3 hydrocarbons from the top of said absorption zone and recovering hydrocarbons having at least 4 carbon atoms to the molecule in each cycle by partial condensation of the overhead from each still prior to the passage of said combined overhead to the absorption-zone.

GEORGE L. MATEER. WALTER H. RUPP. 

